vein and the ductus venosus undergo atrophy and obliteration after birth, and form respectively the ligamentum teres and ligamentum venosum of the liver.

The Parietal Veins.The first indication of a parietal system consists in the appearance of two short transverse veins, the ducts of Cuvier, which open, one on either side, into the sinus venosus. Each of these ducts receives an ascending and descending vein. The ascending veins return the blood from the parietes of the trunk and from the Wolffian bodies, and are called cardinal veins. The descending veins return the blood from the head, and are called primitive jugular veins(Fig. 477). The blood from the lower limbs is collected by the right and left iliac and hypogastric veins, which, in the earlier stages of development, open into the corresponding right and left cardinal veins; later, a transverse branch (the left common iliac vein) is developed between the lower parts of the two cardinal veins (Fig. 479), and through this the blood is carried into the right cardinal vein. The portion of the left cardinal vein below the left renal vein atrophies and disappears up to the point of entrance of the left spermatic vein; the portion above the left renal vein persists as the hemiazygos and accessory hemiazygos veins and the lower portion of the highest left intercostal vein. The right cardinal vein which now receives the blood from both lower extremities, forms a large venous trunk along the posterior abdominal wall; up to the level of the renal veins it forms the lower part of the inferior vena cava. Above the level of the renal veins the right cardinal vein persists as the azygos vein and receives the right intercostal veins, while the hemiazygos veins are brought into communication with it by the development of transverse branches in front of the vertebral column (Figs. 479,480)

FIG. 478 Scheme showing early stages of development of the inferior vena cava. (See enlarged image)

Inferior Vena Cava.The development of the inferior vena cava is associated with the formation of two veins, the subcardinal veins (Figs. 477,478). These lie parallel to, and on the ventral aspect of, the cardinal veins, and originate as longitudinal anastomosing channels which link up the tributaries from the mesentery to the cardinal veins; they communicate with the cardinal veins above and below, and also by a series of transverse branches. The two subcardinals are for a time connected with each other in front of the aorta by cross branches, but these disappear and are replaced by a single transverse channel at the level where the